Special Addresses

 Directed Broadcast Address

• A directed broadcast address is used by hosts to send data to all devices on the specific subnet or specific network in direct broadcast addresses.
• The entire host portion of the address is populated with binary ones.
• For Example: For Network 172.31.0.0, directed broadcast address will be 172.31.255.255
• As it is a class B address, the first two octets denote network, and the last two octets denotes host portion of the address. So, the host portion is filled with binary ones.
• Routers can be configured to route directed broadcasts, but by default, directed broadcasts are not routed from one physical interface to another physical interface or from one VLAN to another VLAN.
• As there are hacking utilities that you can download and use to launch denial of service attacks or DDoS attacks by using directed broadcasts and thus for security reasons, it's recommended that the forwarding of directed broadcasts be disabled.
• Smurf is an example of an application that allows you to launch denial of service attacks using directed broadcasts.

Local Broadcast Address

• Local Broadcast Address is used to communicate with all devices on the local Network. The address is populated with binary 1’s so it looks like this
• 11111111.11111111.11111111.11111111
• 255.255.255.255
• This address is used for example by a host when requesting an IP Address from DHCP server.
• The host doesn't have an IP Address yet, so it doesn't know which subnet it belongs to, it also doesn't know the IP Address of the DHCP server so it will send a broadcast to this address the Local Broadcast Address to request an IP Address from a DHCP server.
• DHCP is Dynamic host Configuration Protocol and it's a service that provides IP Addresses dynamically to devices such as PCs, phones, iPads, IP Telephones, and other devices that you may encounter on a Network. So rather than you manually configuring IP Addresses on your iPhone or your iPad or an IP Telephone or a PC for that matter, you would allow that device to boot up sends out a broadcast to the Local Broadcast Address and the DHCP server hears that broadcast and then allocates an IP Address to the host from a pool of IP Addresses or scope of IP Addresses.
• Be aware that Local Broadcast Addresses are always dropped by Layer 3 devices such as routers and Layer 3 switches you can override that functionality by configuring what is called DHCP forwarding also called DHCP relay on your routers and switches.

Local Loopback Address

• It is very useful for testing; a device can send a message to itself and this helps verify or make sure that the TCP/IP stack is installed correctly on that machine.
• A typical Loopback IP Address is 127.0.0.1 but remember please that anything in the 127 range is deemed a Loopback Address.

Note: Routers and switches also have Loopback Addresses and that's not the same concept as the Loopback Address that we're discussing here.